Examples of treatable streams include, among others, ventilation makeup air, ambient air, air from stripping and off-gassing operations, soil vapor extortion (SVE), airborne matter (e.g. organic particulate, biogenic and microbial matter) and process vent gas, wastewater treatment off-gas, liquid effluents (e.g. wastewater, industrial and agricultural runoff) containing at least one undesirable or otherwise unwanted compound. Moreover, this application presents a holistic approach to the design of the high performance photo- and thermocatalytic systems that possess:
ixe2x80x94Rapid species mass transfer to and from the active sites of the catalyst.
iixe2x80x94Uniform transport of thermal and radiant energy to the active sites of the catalyst.
iiixe2x80x94Decoupling of the conversion efficiency from process intrinsic energy efficiency.
ivxe2x80x94Minimal pressure drop.
As environmental regulations become progressively more stringent, new techniques and approaches are needed for dealing with difficult contaminants. For example, the required destruction and removal efficiencies (DREs) for some environmental pollutants, such as toluene disocyanate (TFD), dioxin, dibenzofurans and polychlorinated biphenyls (PCBs) are extremely high. Conventional methods such as carbon adsorption or liquid scrubbing are not a complete remediation solution due to the fact that they simply transfer contaminants from one medium (i.e. water or air) to another (i.e. solid carbon or scrubbing liquid). On the other hand, incineration and catalytic thermal oxidation present their own limitations. For example, the widespread production and use of chlorinated compounds in the industrially developed countries has resulted in large amounts of halogenated organic contaminants to seep into the soil, water and air. Incineration and even thermocatalytic oxidation of wastestreams containing halogenated compounds in many cases produce emission of products of incomplete combustion (PIC) such as dibenzofurans, dioxin and other pollutants that are known or suspected carcinogens. It is to be understood that in the terminology of this application xe2x80x9ctarget species/compoundsxe2x80x9d denote those entities contained within the contaminated stream that are targeted for complete destruction and removal.
The past two decades has seen rapid growth and promulgation of new remediation technologies. In particular, a class of pollution control technologies known as the advanced oxidation processes (AOPs) has been the focus of much research and development. Among AOPs, those that employ ultraviolet (UV) radiation in conjunction with active oxidants (i.e. ozone, hydrogen peroxide, hydroxyl radical, superoxide ion radical, etc.) to accomplish mineralization of the target organic contaminants are of special interest. Generally, UV/AOPs are characterized with respect to the type of either the catalyst and chemical reactions involved (i.e. homogeneous vs. heterogeneous) or light source employed (i.e. solar vs. artificial).
In general, UV/AOPs for treatment of the hazardous organic contaminants (HOCs) in fluids (both gas- and liquid-phase) comprise the following steps:
In the first step, an organic contaminant (hereafter called xe2x80x9cprimary reactantxe2x80x9d or xe2x80x9ctarget compoundxe2x80x9d) that is adsorbed on the catalyst surface or resides within the fluid reacts to form products (hereafter termed xe2x80x9cintermediatexe2x80x9d or xe2x80x9csecondaryxe2x80x9d products).
In the next step, the secondary products react to form other products (hereafter called xe2x80x9ctertiary productsxe2x80x9d or xe2x80x9cfinal productsxe2x80x9d) that can be retarded as more benign, safer, or less detrimental to health and environment. The tertiary products are formed through a sequence or stepwise reaction scheme and an effective wave to obtain tertiary or final products is to use specially engineered catalytic reactors disclosed in this document.
It is generally recognized that the UV-based AOPs do not universally enjoy high process energy efficiencies. This realization has motivated many researchers to test the concept of integrated or hybrid processes. In this approach, several processes are combined to produce a hybrid system that is capable of treating contaminants in the waste stream at much higher overall process energy efficiency and reduced life-cycle costs than each of individual processes, alone. This is especially true in applications where the initial concentration of the target compound may vary wildly in the course of the treatment process.
A good example is ethanol emission (in air) from some pharmaceutical product dryers. Ethanol concentration in the product dryer varies during a typical cycle by two orders of magnitude. Also, hybrid processes can be used in certain applications where valuable chemicals (e.g. acrylonitrile monomer, solvents, etc.) are emitted in the effluent that can be recovered. Yet another example involves treatment of the energetic materials. It is known that the photocatalytic treatment and mineralization of 2,4,6-trinitrotoluene (TNT) in aqueous media is difficult. However, once partially oxidized, many microorganisms can readily metabolize the partial oxidation products. Here, a UV/AOP is combined with another treatment process (i.e. biological) to achieve a much higher process efficiency. Examples of surrogate processes employed in the prior art include bioremediation, electron beam, thermocatalytic oxidation, activated carbon or synthetic adsorbents, UV/H2O2 and UV/O3, to name just few. Alternatively, performance improvement can be made at the catalyst/support level, using multifunctional catalytic media, i.e. capable of acting as both photocatalyst and thermocatalyst.
It is to be understood that in the terminology of this application, xe2x80x9cmediaxe2x80x9d or xe2x80x9ccatalytic mediaxe2x80x9d denotes the combination of photocatalyst(s) and its/their supporting base material(s). Most base material(s) of the prior art simply provide(s) a structural support for the active catalyst(s) used and do not normally partake in the reactions or provide other known functions. Examples include, but not limited to, U.S. Pat. Nos. 4,892,712, 4,966,759 and 5,032,241 to Robertson et al., U.S. Pat. No. 5,126,111 to Al-Ekabi et al.; and U.S. Pat. No. 5,035,784 to Anderson et al. However, it is possible to have a multifunctional media that is both photocatalytically and thermocatalytically active. The rationale for using a multifaceted media will now be described.
Consider a UV/AOP that employs a high power light source such as a medium-pressure mercury lamp (MPML). MPMLs generate large amounts of thermal radiation, at relatively high temperatures. Even when a low-pressure mercury lamp (LPML) is used as the source of UV light, considerable amount of low-level waste heat is given off. For example, according to vendor specifications, a standard 65 W Voltare(copyright) lamp (G64T5VH), converts less than 40% of the input electrical power to emitted light in the form of 254-nm radiation. The electric to UV energy conversion efficiency is lower yet for fluorescent black light (less than 25%) and medium pressure mercury lamps (less than 15%).
It is generally recognized that only a very thin layer on the photocatalyst surface can actually be excited to enter photocatalytic reactions. For most active photocatalysts, the physical thickness of this layer or skin does not exceed few microns. This is due to the fact that UV radiation is completely absorbed within a skin only few microns thick on the exposed photocatalyst surface. On the other hand, thermal radiation can penetrate deep into the supported catalyst and base material. The fact that most target species can also be adsorbed into the deep layers of the photocatalytic media (inaccessible to UV but affected by thermal radiation and heat) encourages the use of multifunctional catalysts capable of utilizing both heat and light emitted by medium and high pressure UV lamps. Thus, a multipurpose catalyst can comprise a base material that acts as both a thermocatalyst as well as support structure for the photocatalyst. Alternatively, a dual catalyst may be used that can function as both thermocatalytic and photocatalyst simultaneously. It is also possible to implement a thermocatalyst and a photocatalyst separate but together, in series.
The use of combined photo- and thermocatalytic action as in an integrated media is known in the prior art. Examples include Muradov, N. Z., Tabatabaie-Raissi, A., Muzzey, D., Painter, C. R. and M. R. Kemme, Solar Energy, 56, 5 (1996) 445-453; and Fu, X., Clark, L. A., Zeltner, W. A., and M. A. Anderson. J. of Photochemistry and Photobiology, A: Chemistry 97 (1996) 181-186, among others. Muradov et al. describe a photo/thermocatalytic method for selective oxidation of airborne volatile organic compounds (VOCs) including nitroglycerin, ethanol and acetone. The light source used was a low-pressure mercury lamp. The catalytic media employed was TiO2 modified with silico-tungstic acid (STA) and platinum. Fu et al. describe photocatalytic degradation of ethylene in air at elevated temperatures over sol-gel derived TiO2 and platinized TiO2 particulates, irradiated with a fluorescent black light lamp. Both studies report improved performance at elevated reaction temperatures without platinization of the photocatalyst.
The use of bandgap semiconductors such as titania (TiO2), ZnO, ZrO2, CdS, etc. and their various modified forms as the gaseous and aqueous phase photocatalysts is well known in the prior art. For example, TiO2 particles (anatase crystalline form, in particular) are readily excited upon exposure to near UV radiation (wavelengths below approximately 400 nm) producing electron/hole (e31/h+) pairs on the semiconductor surface. The recombination of exe2x88x92/h+pairs has the resulting effect of reducing the process quantum efficiency. The recombination can occur either between the energy bands or on the semiconductor surface.
It has long been recognized that certain materials such as noble metals (e.g. Pt, Pd, Au, Ag) and some metal oxides (e.g. RuO2, WO3, and SiO2) facilitate electron transfer and prolong the lenth of time that electrons and holes remain segregated. The electrons and holes act as strong reducing and oxidizing agents that cause break down of the target compounds via formation of active radicals on the photocatalyst surface. The following groups of reactions describe the excitation of titania leading to the generation of active radicals:
TiO2+hvxe2x86x92h+vb+exe2x88x92cbxe2x80x83xe2x80x83(i)
h+vb+OHxe2x88x92adxe2x86x92xe2x80xa22OHadxe2x80x83xe2x80x83(ii)
exe2x88x92cb+(O2)adxe2x86x92(Oxe2x80xa22)adxe2x80x83xe2x80x83(iii)
(O2xe2x88x92xe2x80xa2ad+H2Oxe2x86x92OHxe2x88x92ad+(HOxe2x80xa22)adxe2x80x83xe2x80x83(ii)
h+vb+exe2x88x92cbxe2x86x92heat (recombination)xe2x80x83xe2x80x83(v)
Reaction (a) occurs within the TiO2 lattice. When TiO2 absorbs a UV photon, represented by hv, having an energy equal to or greater than its bandgap energy, electrons (exe2x88x92cb) shift to the conduction band, and positively charged xe2x80x9cholesxe2x80x9d (h+vb) remain behind in the valence band. Energy is related to wavelength by Planck""s equation:
xe2x80x83E=hc/xcex
Where:E is the bandgap energy (eV), h is Planck""s constant (6.6256xc3x9710xe2x88x9234 Js) and c refers to the velocity of light (2.998xc3x971010cm/s), and xcex is the wavelength (nm) of radiation.
Assuming bandgap energy of 3.1 eV for TiO2, a threshold wavelength of about 400 nm is obtained. TiO2 will absorb light having a wavelength equal to or lower than this value. Once holes and electrons are photogenerated they move about the crystal lattice freely in a manner described as the xe2x80x9crandom walk.xe2x80x9d The random walk results in the electrons and holes either recombining (thermalizing) per equation (v) or reaching the surface of the catalyst to react with adsorbed species and produce reactive radicals as indicated by equation (ii), (iii) and (iv).
An important factor in controlling the rate of electron-hole recombination on the photocatalyst surface is the size of catalyst particles. The smaller these particles are, the shorter the distance that charge carriers must travel to reach the surface and the larger the exposed catalyst surface area is. Photocatalysts having X-ray diameter of only a few nanometers and BET surface area of many 100 s m2/g are commercially available (e.g. ST-01 and ST-31 grades titania produced by Ishihara Sangyo Kaisha, LTD of Japan).
The rate of recombination of holes and electrons is a function of the catalyst surface irradiance. Prior art teaches that higher the surface irradiance, the greater the rate of recombination of electrons and holes (Egerton, T. A., King, C. J., J. Oil Col. Chem. Assoc., 62 (1979) 386-391). Prior art also teaches that only one of the process (ii) or (iii+iv ) is the rate-limiting step. The process involving the other radical completes the reaction and preserves the overall charge neutrality. Thus, it is generally recognized that the hydroxyl radical formation is the rate-limiting step. The rate of surface reactions will then be equal to r=k(cxe2x88x92d)[h+vb]. The rate of hole formation is kuqiwhere qi denotes catalyst surface irradiance (quanta/s/cm2). The rate of electron-hole recombination is then ke[h+vb][exe2x88x92cb]=ke[h+vb]2. When qi is high, a large number of electrons and holes will be generated, and Egerton and King have already shown that: r=kqixc2xd. At low values of qi when surface concentration of holes, [h+vb], is relatively small, the recombination term will be negligible and r=kaqi. The surface irradiance value (hereafter called xe2x80x9cEgerton-King thresholdxe2x80x9d) at which the reaction rate transition from qi to qixc2xd(1 to xc2xd dependency) occurs is qEK=2.5xc3x971015 quanta/s/cm2 (at xcex=335, 365 and 404 nm).
The qEK can be calculated for two commonly used UV light sources (i.e. low- and medium-pressure mercury lamps). For the LPMLs and MPMLs qEK is approximately equal to 1.95 mW/cm2 (for xcex=254 nm) and 1.36 mW/cm2 (for xcex=365 nm), respectively. In order to limit the rate of recombination of electrons and holes and maximize the photoreactor performance, it is necessary to limit the catalyst surface irradiance to levels at or below the Egerton-King threshold. The rate of surface reactions, r, is proportional to qim, where m varies between xc2xd and 1. To increase the rate of surface reactions for target pollutants, it may be necessary to allow qi to exceed qEK under certain conditions. Therefore, in a practical situation, the requirement for an efficient utilization of the photogenerated charge carriers must be balanced against the need for optimum rate of the surface reactions involving the primary and secondary reactants that produce desirable final products. In general, this requires a careful photoreactor design that allows uniform irradiation over all photocatalytic surfaces at a level that is as close to qEK as possible and optimum rate of conversion of surface-borne target species to desirable final products.
Just like radiation and heat transfer, transport of the primary reactants to and final products from the catalyst surface affect the photoprocess performance. The reactor engineering is closely coupled to the choice and configuration of the media and the type of light source used. A proper photoreactor design should provide for uniform irradiance on all catalytic surfaces as well as effective species mass transport to and from the catalyst active sites. Mass transfer limitations affect the process efficiency, as all target species must reach the active/activated catalyst surface before any reaction can occur. For process streams containing very low concentration of contents, the transport effects are even more pronounced. In general, photoreactor designs fall into one of the following three categories:
1. Most photocatalytic reactors/processes of the prior art belong in here. The Category I photoreactors possess good mass transfer but generally poor radiation field characteristics.
FIG. 1a, 1b, 1c depict several examples from prior art depicting photocatalyst-coated monolith, photocatalyst-coated panel, and baffled annular photoreactor, respectively. Other examples include Australian Patent PH7074 to Matthews; U.S. Pat. No. 3,781,194 to Juillet et al.; U.S. Pat. No. 4,446,236 to Clyde; U.S. Pat. No. 4,774,026 to Kitamori et al.; U.S. Pat. Nos. 4,888,101 and 5,736,055 to Cooper; U.S. Pat. Nos. 4,892,712, 4,966,759 and 5,032,241 to Robertson et al.; U.S. Pat. No. 5,126,111 to Al-Ekabi et al.; U.S. Pat. No. 5,045,288 to Raupp et al.; U.S. Pat. No. 5,069,885 to Ritchie; U.S. Pat. No. 5,480,524 to Oeste; U.S. Pat. No. 5,564,065 to Fleck et al.; U.S. Pat. No. 5,683,589 to de Lasa et al.; U.S. Pat. No. 5,790,934 to Say et al.; and U.S. Pat. No. 5,030,607 to Colmenares, to name just a few.
2. Poor mass transfer but mostly uniform catalyst surface irradiance, e.g. annular photoreactor design (no internals, catalyst coated on the outer wall).
3. Poor mass transfer and non-uniform catalyst surface irradiance, e.g. externally lit annular photoreactor (no internals, catalyst coated on the inner wall).
As noted before, a good photocatalytic reactor design should provide for a uniform near qEK catalyst surface irradiance and temperature as well as no mass transfer limitations. This requires considerable process and reactor optimization effort prior to scale-up. Experimental techniques involving the measurement of the radiative properties of materials including photocatalysts are generally very complex and time consuming. Likewise, computational methods for analyzing radiative exchange among surfaces and between surfaces and gases even under the simplest of conditions are very difficult to execute. This so because the equation of transfer, in general, is of the complex integro-differential form and very difficult to solve. Other complexities including chemical reactions, species mass transfer, etc. further complicate photoprocess/reactor analysis and optimization. Therefore, it is not surprising that the prior art offers very little in the way of photocatalytic process and reactor analysis, modeling, optimization and scale-up. When it comes to the photocatalytic reactor and process engineering and design, the prior art methodologies are mostly pseudo-quantitative, semi-empirical and intuitive, in nature.
For example, it has long been recognized that providing means for generating and enhancing turbulence in the flow generally improves species mass transfer to and from the catalyst surface active sites. An examination of the prior art reveals that many articles such as ribs, fins, pleats, beads, chips, flaps, strips, coils, baffles, baskets, wires, etc. have been conceived, used and patented for generating mixing and turbulence in the flow and generally improve mass transfer characteristics of the reactors. Thus, using flow agitating articles or xe2x80x9cinternalsxe2x80x9d to enhance the contaminant mass transfer to the catalyst surface is more or less intuitive. But, the effect of internals or xe2x80x9cturbulatorsxe2x80x9d on the radiation field within the photoreactor seems to be less obvious and seldom fully appreciated. Often, methods used in the prior art to eliminate mass transfer intrusions adversely affect the extent and uniformity of radiation received on the catalyst surface, within the same photoreactor. One example is the annular photoreactor having internal baffles such as one shown in FIG. 1c. The U.S. Pat. No. 5,683,589 (de Lasa et al.), U.S. Pat. No. 5,069,885 (Ritchie), U.S. Pat. No. 5,116,582 (Cooper), and U.S. Pat. No. 5,790,934 (Say et al.) are all variations of this basic configuration. The catalyst surface irradiance for the photoreactor configuration of FIG. 1c has been carried out by the subject inventor and results are given in FIG. 2.
Results of FIG. 2 indicate that, if internals must be used to improve mass transfer, it is more advantageous to design the photoreactors in such a way that the bulk of catalyst resides on the reactor wall. This requirement limits the number and proximity of internals, in general, and baffles, in particular, that can be incorporated into the photoreactor. It can be seen that for the baffle spacing smaller than one baffle diameter (see U.S. Pat. No. 5,683,589 to de Lasa et al. and U.S. Pat. No. 5,790,934 to Say et al.), the surface irradiance (as a fraction of the lamp""s radiosity) is lower on reactor wall than the baffle surface. Furthermore, results of FIG. 2 indicate that the point of diminishing return with respect to the magnitude and uniformity of the surface irradiance is reached at inter-baffle spacing, L, of about 10 times the sleeve diameter (Di). The that the baffle splacing equal or greater than L=10Di is necessary for achieving a uniform irradiance results in the wall irradiance levels that are well above the qEK. Moreover, the L/Di=10 requirement results in inter-baffle distances that are unsuited to proper fluid mixing. These and other effects combine to make the use of most internals or turbulators generally undesirable.
Another important but poorly understood phenomenon within the photocatalytic reactors of the prior art is the light refraction and reflection effect. FIG. 3 depicts an annular photoreactor with three linear UV lamps, 120xc2x0 apart, along the reactor axis. FIG. 4a-4b depict the lateral variation of the wall irradiance as a function of the packing radius, rp. All three lamps are lit and data are shown for two rp/ro values (0.333 and 0.452) and a range of baffle spacing, denoted by L/ro, from 0.76 to 6.10. On the same graph, the analytical predictions for the lamp as a diffuse line source emitter are also given. The measured wall irradiance dips at all locations having shortest radial distance to the lamp axis. This effect is due to the refraction and blocking of UV rays from the posterior lamps. When the refraction effects are all accounted for, the experimental data are in good agreement with the analytical and model predictions. This is shown in FIG. 5 for one of the baffle spacing of the arrangement of FIG. 4a, i.e. L/ro=6.10. This example clearly shows that refraction and reflection of light is likely to affect irradiance distribution within the catalytic matrix of several photoreactor designs of the prior art such as the U.S. Pat. Nos. 4,892,712, 4,966,759 and 5,032,241 (Robertson et al.) and U.S. Pat. No. 5,126,111 (Al-Ekabi et al.). It can now be appreciated that the configuration of the catalytic media and design of the photocatalytic and thermocatalytic reactors must be kept as simple as possible. This requirement is in addition to ones discussed before (i.e. having good mass transfer and radiation field characteristics).
Moreover, a photoreactor design that yields a uniform irradiance distribution over all its catalytic surfaces, does not lend itself to mass transfer intrusions and has a simple design that is readily scalable, can still be affected by low process energy efficiency. This is so because, in one-pass reactors, the process energy efficiency is coupled with the conversion efficiency (or process DRE). When very high process DREs are required, the transport effects lead to process energy efficiencies that are well below the maximum realizable. This so-called xe2x80x9ccoupling effectxe2x80x9d adds another complexity to the design of high-performance photocatalytic and thermocatalytic reactors. Thus, one object of the present invention is to teach a novel method for mitigating the adverse effects of coupling on the performance and energetics of single-pass photocatalytic. thermocatalytic or combined photo- and thermocatalytic reactors.
An examination of the prior art reveals that six distinct types of catalytic media arrangement have been used, to date. For the sake of discussions here, they are termed as the Type 0, Type I, Type II, Type III, Type IV and Type V, of which Types 0-II and IV are substantially photocatalytic and Types III and V are substantially thermocatalytic, albeit multifunctional media.
In Type 0 photocatalyst/support configuration, a suitable catalyst such as titania is used in colloidal form without any support or base material(s). Examples of Type 0 media include, among others, U.S. Pat. Nos. 5,554,300 and 5,589,078 to Butters et al.; U.S. Pat. Nos. 4,888,101 and 5,118,422 to Cooper et al.; and U.S. Pat. No. 4,861,484 to Lichtin. A sub-category of Type 0 media includes, among others, U.S. Pat. No. 5,580,461 (Cairns et al.). Cairns, et al. employ a combined process that includes, in addition to colloidal titania photocatalysis, a surrogate process based on the use of adsorbent material. The contaminated fluid is first contacted with a articulate adsorbent material that physically adsorbs the target compound. The contaminant-loaded adsorbent is then separated from the fluid and brought into contact with aqueous slurry of a suitable photocatalyst. The use of adsorbent material implies, implicitly, that the technique is more suited to treatment of processes in which the adsorption of target species on the photocatalyst surface is the rate-limiting step. This is not generally the case, especially in the vapor-phase processes where the rate of reaction for one or more surface bound species (primary or secondary reactants) control the overall rate of the reaction and final process outcome. It is therefore desirable to simplify the treatment process by eliminating the surrogate adsorbent in favor of a multifunctional catalytic media (catalyst and support combination) that is both a good adsorbent as well as a good photocatalyst.
In Type I photocatalyst/support arrangement, the catalyst (often a modification of the anatase crystalline form of TiO2) is immobilized or bonded onto a ceramic, glassy (e.g. fiberglass mesh, woven glass tape, etc.) or metal oxide (e.g. silica gel), metallic (e.g. stainless steel), or synthetic polymeric (e.g. plastic) substrate. Examples of Type I media include, among others, U.S. Pat. No. 5,564,065 to Fleck et al.; U.S. Pat. No. 5,449,443 to Jacoby et al.; U.S. Pat. No. 5,045,288 to Raupp et al.; U.S. Pat. No. 5,069,885 to Ritchie; U.S. Pat. No. 4,446,236 to Clyde; U.S. Pat. No. 5,736,055 to Cooper; U.S. Pat. No. 5,683,589 to de Lasa et al.; U.S. Pat. No. 5,790,934 to Say et al.; and U.S. Pat. No. 5,374,405 to Firnberg et al.
In Type II media configuration, impregnated glassy mesh/matrix or porous ceramic monolith or beads, metallic and metal oxide substrates (in the form of plates, beads, etc.) are employed as the photocatalyst support to which titania is bonded utilizing a method known as the xe2x80x9csol-gel technique.xe2x80x9d There are many variations, but, a typical process for preparing colloidal sols and corresponding media is discussed in xe2x80x9cPhotocatalytic Degradation of Formic Acid via Metal-Supported Titania,xe2x80x9d H. Y. Ha and M. A. Anderson. J. of Environmental Engineering, March, 1996, pp. 217-18, the contents of which are incorporated herein by reference. First, a solution of titanium isopropoxide mixed with dilute nitric acid in a ratio of H2O/Ti(i-Pro)4/70% HNO3=300/30/20 ml is refluxed at 80 degrees centigrade for 3 days. The resulting colloid is then concentrated with a vacuum rotary evaporator. The final titania concentration of the colloid becomes 1.06 mol/L at pH 0.8. The media support used were stainless steel 304 plates and tin (IV) oxide-covered glass. The stainless-steel plates were pretreated by firing at 450 degrees centigrade for 2 hours to produce a metal oxide layer. A PMW spinner system was used to produce uniform titania layers on the support. The support was spun at 2500 rpm for 30 seconds. The coated gel was first dried at room temperature and then fired at a temperature that may vary between 300 and 600 degrees centigrade with a heating rate of 3 degrees centigrade per minute. Typical dwell times were about 2 hours. The process is repeated until the desired catalyst thickness is obtained.
Type II catalyst/support examples include, but not limited to, U.S. Pat. Nos. 4,892,712, 4,966,759 and 5,032,241 all to Robertson et al.; U.S. Pat. No. 5,126,111 to Al-Ekabi et al.; and U.S. Pat. No. 5,035,784 to Anderson et al. In Type I and Type II arrangements, the substrate has no known function other than providing physical and structural support for the photocatalyst.
Type II catalyst/support configuration is a variation of the Type II media that involves synthesis and use of metal oxide aerogels, most prominently SiO2 aerogels doped or co-gelled with other transition metal oxides such as titania to produce photochemically active catalyst/support material. There are many methods and variations of the basic technique used for preparing high porosity metal oxide aerogels. In general, preparation of metal oxide aerogels and porous glasses comprise a two step process in which a condensed metal oxide intermediate is formed. From this intermediate compound aerogels are prepared having any desired density, clarity and UV transparency, thermal insulation capacity, moisture and mechanical stability.
Two general reactions have been used to make earlier metal oxide aerogels. In the process of U.S. Pat. No. 2,249,767 to Kistler, first a metal alkoxide is hydrolyzed by reacting with water and an alcohol in the presence of a reagent (e.g. NH4OH). Second, the hydrolyzed metal undergoes a condensation reaction to form a metal oxide gel, from which an aerogel is made by supercritical fluid extraction of the solvents. An improvement to the Kistler""s method is given by the single-step sol-gel process of the U.S. Pat. No. 3,672,833 to Teichner et al. Teichner""s method, employs a silicon alkoxide tetramethoxysilane or tetraethoxysilane which is hydrolyzed by one to ten times stoichiometric quantity of water with an alcohol in an acidic, neutral or alkali environment. This is followed by the condensation reaction in which the hydrolysis products polymerize to form a wet gel. In Teichner""s method, the alcohol is removed directly from the wet gel at above supercritical pressure and temperature point of the alcohol. It should be noted that any metal that can form an alkoxide, which includes essentially the entire periodic Table of elements, could be used to make an aerogel. Examples include: silicon, germaniium, zirconium, titanium, iron, magnesium, boron, cerium, lithium and aluminum, to name just few.
Further improvement upon the techniques developed by Kistler and Teichner has been made recently through many new syntheses methods. Examples include, among others, U.S. Pat. No. 5,030,607 to Colmenares; U.S. Pat. Nos. 5,275,796 and 5,409,683 to Tillotson et al.; U.S. Pat. No. 5,538,931 to Heinrichs et al.; U.S. Pat. No. 5,718,878 to Zhang; U.S. Pat. No. 5,759,945 to Carroll et al.; U.S. Pat. No. 5,766,562 to Chattha et al.; and U.S. Pat. No. 5,753,305 to Smith et al. As an example, the properties of the low-density silica aerogels made by method of the U.S. Pat. No. 5,409,683 (Tillotson et al.) is described and incorporated here by reference in its entirety. The density of the silica aerogels prepared by this method varies typically between approximately 0.0015 g/cm3 and 0.8 g/cm3. Representative refractive index of the Tillotson silica aerogels are in the range of 1.0005 and 1.170 when measured at a wavelength of 632.8 nm. Light transmittance is typically greater than 85% at 632.8 nm. For a monolithic silica aerogel, 2 cm thick, a bulk density of 0.05 g/cm3 and prepared by the method of U.S. Pat. No. 5,409,683, the light transmittance at xcex=400 nm is typically 45%. The porosity, expressed as the percentage of open space within the total volume, falls in a range between 64% and 99.9%. The specific surface area of these silica aerogels is in the range of 450 to 100 m2/g. The properties of silica aerogels given here by reference to the U.S. Pat. No. 5,409,683 to Tillotson et al. are also typical of other metal oxide aerogels (e.g. titania) prepared by similar techniques.
A typical Type III media most useful to the practice of the present invention can be made by methods of the U.S. Pat. No. 5,409,683 to Tillotson which is incorporated here by reference. In Tillotson""s two-step method, a high purity metal (e.g. silicon, titanium, zirconium) alkoxide is mixed with a hydrolysis rate reducing alcohol (such as methanol, ethanol or propanol), an additive (e.g. acetylacetone, acetic acid and hydrogen peroxide) and a substoichiometric amount of water to form a solution. If silicon metal is used the suitable alkoxide is tetramethoxysilane (TMES). Likewise, for titanium metal, the desirable alkoxide is titanium isopropoxide. The metal alkoxide solution is then reacted with a suitable acid catalyst (e.g. hydrochloric acid) to form an oligomeric mixture of a partially condensed metal intermediate and a reaction produced alcohol. This is followed by the removal of alcohol by distillation and evaporation. The next step involves adding a nonalcoholic solvent such as acetonitrile or acetone to the partially condensed metal intermediate to form a non-alcoholic solvated condensed metal intermediate which is then reacted with a second catalyst (ammonia or fluoroboric acid) and mixed. The amount of catalyst regulates the pH of the solution and determines the rate of gel formation. After mixing is completed, the condensed metal oxide product is cast, that is, poured into a mold to form a wet gel. The gelation takes about 72 hours and carried out at room temperature. The nonalcoholic solvent and any reaction-generated alcohol is then removed by supercritical extraction using liquefied carbon dioxide, chlorofluorocarbons (freons) or propane. More recently, methods have been developed for preparation of both bulk and thin film aerogels in which the gel drying is carried out under subcritical conditions (Jochen Fricke, xe2x80x9cSuperexpansive Gels,xe2x80x9d Nature, vol. 374. pp. 409-410, 1995). Another important development involves rapid aging technique for aerogel thin films (U.S. Pat. No. 5,753,305 to Smith, et al.).
An important application of the metal oxide aerogels is their use as heterogeneous catalyst and support structure for chemical processes involving oxidation, epoxidation, hydrogenation, reduction, synthesis, etc. As such, co-gelled metal oxide aerogels such as titania-silica aerogels and transition metal aerogel-supported catalysts (e.g. platinum, nickel, cobalt and copper supported on silica aerogel) are well known in the art. For example, U.S. Pat. No. 5,538,931 to Heinrichs, et al. teaches a process for preparing a supported catalyst comprising a transition metal such as palladium or platinum on an aerogel (e.g. silica) that is most useful as a hydrogenation catalyst. U.S. Pat. No. 5,766,562 to Chattha et al. discloses a method for preparing titania aerogel supported precious metal (e.g. platinum, rhodium) catalyst useful for the automotive exhaust gas (NOz, hydrocarbons and carbon monoxide) emission control. U.S. Pat. No. 5,030,607 to Colmenares teaches a method for preparation of UV light-transparent silica aerogels doped with photochemically active uranyl ions (UO2++) for photocatalytic synthesis of short chain hydrocarbons in a fluidized bed photoreactor.
In Type IV photocatalyst/support media, a photocatalyst (e.g. doped and undoped modifications of TiO2, CdS, etc.) is deposited by bonding or cementing onto the fabric of a modified or unmodified natural or synthetic polymer material. Examples for polymers of natural origin (or biopolymers) include wood, paper, kozo, gampi, Kraft lignin, and woven cotton, kenaf, linen, wool, etc. (U.S. Pat. No. 5,246,737 to Muradov and U.S. Pat. Nos. 5,604,339 and 5,744,407 to Tabatabaie-Raissi et al.).
Finally, the Type V media includes the broad field of moderate-temperature (approximately 150-350xc2x0 C.) thermal oxidation catalysts. Of particular interest to practice of the present invention is a sub-class of the moderate temperature thermal oxidation catalysts that include supported transition metal oxide catalysts and cation modified zeolites as dual function sorbent/catalyst media. For example, U.S. Pat. No. 5,414,201 to Greene discloses a combined sorbent/catalyst dual function media which removes dilute VOCs, both halogenated and otherwise, from air at room temperature, and then acts as a catalyst at higher temperatures (350xc2x0 C.) to both desorb and oxidize trapped VOCs. Due to their microporous crystalline structure, various forms of zeolites like zeolite A (3A, 4A and 5A), Faujasites (zeolites X and Y) and Pentasils (ZSM-5 and Silicalite) have been widely used as commercial adsorbents. Two dual function media, Crxe2x80x94Y and Cr-ZSM-5 as well as metal-loaded Coxe2x80x94Y zeolite catalyst, prepared by Greene, Prakash and Athota (J. of Applied Catalysis B: Environmental 7 (1996) 213-224), and Ramachandran, Greene and Chattejee (J. of Applied Catalysis B: Environmental 8 (1996) 157-182), are given below and included here by reference in their entirety.
Crxe2x80x94Y is made by exchanging NH4xe2x80x94Y with chromium nitrate solution containing 1.5 gram of chromium nitrate in one liter of distilled water maintained at a pH of 4 for 72 hours. NH4xe2x80x94Y is prepared by exchanging 15-20 grams of Hxe2x80x94Y (LZ-Y-84 from UOP, Si/Al=2.5, 20 wt % alumina as binder) with 2.24 mol/l ammonium chloride solution for 2 hours. Cr-ZSM-5 is made by exchanging NH4-ZSM-5 with chromium nitrate solution containing 2.3 grams of chromium nitrate in one liter of distilled water at 50xc2x0 C. for 72 hours. NH4-ZSM-5 is prepared by exchanging 15-20 grams of H-ZSM-5 (MFI from UOP, Si/Al=16, 20 wt % alumina as binder) with 2.24 mol/l ammonium chloride solution. After repeated washing, both exchanged catalysts are dried and subsequently calcined at 500xc2x0 C. Typical exchanged chromium loading of the Crxe2x80x94Y and Cr-ZSM-5 catalysts were 0.6 and 0.3 wt %. Typical BET surface area of the Crxe2x80x94Y and Cr-ZSM-5 dual function catalysts were 474 and 388 m2/g.
To prepare Coxe2x80x94Y, about 20 grams of NH4xe2x80x94Y is cobalt exchanged with a solution containing 16 grams of Co(NO3)2.6H2O dissolved in 11 of deionized water. The solution is stirred continuously for 48 hours at 90xc2x0 C. Typical cobalt loading on the zeolite was 1.5 wt %. After the exchange of all the cobalt ions in the cobalt nitrate solution with Hxe2x88x92 ions of the zeolite catalyst, the pellets were thoroughly washed with deionized water, dried at 120xc2x0 C. for 2 hours and then calcined at 500xc2x0 C. for 10 hours. The measured BET surface area of the Coxe2x80x94Y catalyst exceeds 600 m2/g of catalyst.
Still another media useful for the practice of this invention has been disclosed by U.S. Pat. No. 5,720,931 to Rossin for catalytic oxidation of organic nitrogen-containing compounds. Typical catalyst composition comprises a noble or a base metal supported on titania (Degussa P-25R) or zirconia with added promoters such as molybdenum, tungsten, or vanadium. A typical formulation given by EXAMPLE I of the U.S. Pat. No. 5,720,931 is incorporated here by reference, in its entirety, 25 g of Degussa P-25titania powder is slurried in 250 ml deionized water. To the slurry is added 2.9 g of lanthanum nitrate hydrate dissolved in 30 ml distilled water. The slurry is placed in a rotary evaporator at 45xc2x0 C. Water is evaporated from the slurry overnight. The remaining solid is dried at 125xc2x0 C. for 2 hours, then crushed and sieved to 25/60 mesh granules. The granules are then calcined at 450xc2x0 C., for four hours. Approximately 8 g of this granules are slurried in 200 ml distilled and deionized water. To this slurry is added approximately 0.9 g ammonium metavanadate dissolved in 80 ml distilled and deionized water. The slurry is then placed in a rotary evaporator at 60xc2x0 C. and water is completely evaporated. The remaining solids are then dried at 125xc2x0 C., for two hours, then calcined at 450xc2x0 C. for four hours. About 2 g of the resulting granules is slurried in 50 ml deionized water. Then, 0.04 g tetraammineplatinum nitrate dissolved in 25 ml distilled, deionized water is added to the slurry. The slurry is placed in a rotary evaporator at 60xc2x0 C., and the water is evaporated overnight. The resulting material is dried at 125xc2x0 C., for two hours, then reduced in a hydrogen atmosphere for another two hours, at 450xc2x0 C., then calcined at 450xc2x0 C. for two hours. The resulting final product contains approximately 1-wt % Pt, 5-wt % V, 5-wt % La, and remaining TiO2 support.
A further description of photocatalytic patents will now be described:
U.S. Pat. No. 5,790,934 to Say et al. discloses a compact reactor for the photocatalyzed conversion of contaminants in a fluid stream. The reactor includes a support structure with multiple non-intersecting aluminum fins oriented parallel to the general flow direction of the stream. The fins were spray coated with a 1:1 mixture of titanium dioxide photocatalyst and alumina. Several germicidal lamps were inserted into the fins that totaled 148 pieces that were either flat or pleated. The photocatalytic reactor of Say et al. had several alternative designs but all included a large number of flat or pleated fins or baffles at various relative configuration to the light source. Although, it is understood that such a design does present certain advantages with respect to the contaminants mass transfer to the photocatalytic surfaces, it is not at all clear how such configurations can be useful in insuring a uniform irradiance over all cataltic surfaces at or near qEK. Furthermore, no effort was made to decouple the process energy efficiency from the DRE of the target pollutant (formaldehyde vapor). Also, no references are given to the use of multifunctional photo- and thermocatalytic media of the Type III-V configuration.
U.S. Pat. Nos. 4,888,101 and 5,116,582 to Cooper and U.S. Pat. No. 5,736,055 to Cooper et al. disclose several titania-based, substantially of the Type 0 slurry photoreactor designs. In one application, a replaceable cartridge for use in a photocatalytic fluid purification is described. The fluid flows through the cartidge in the presence of light. The cartridge includes a flexible; porous element having titania coating associated with it and a rigid support structure. In another embodiment of the invention, a system for photocatalytic modification of a chemical composition comprising substantially titania entrapped within a layer of Pyrex glass wool interposed between two transparent plates. In yet another embodiment, a photocatalytic slurry reactor is disclosed that is driven by solar or artificial UV illumination. A tubular UV lamp is suspended by an O-ring within a cylindrical reactor jacket, creating an annular region through which a titania slurry is pumped. A helical stainless steel wire wrapped about the bulb acts as a turbulence generator to break up the boundary layer for increased radial mixing.
These processes are substantially Type 0 slurry reactors with generally acceptable mass transfer characteristics but non-uniform irradiance over catalytic surfaces, i.e. category I limitation. No effort was made by these researchers to decouple the process energy efficiency from DRE of the target pollutants. Also, no references are given to the use of multifunctional photo/thermocatalytic media of the Type III-V configuration.
U.S. Pat. Nos. 5,604,339 and 5,744,407 to Tabatabaie-Raissi et al. describe the use of photocatalysts, and in particular titania, as coating on the woody or biopolymeric support materials as an in-situ treatment technique to prevent emission of harmful volatile organic compounds such as formaldehyde, xcex1-pinene, xcex2-pinene and limonene from emitting surfaces. This invention is strictly an in-situ application and no description is made of ex-situ treatment of airborne contaminants or process vent gases utilizing a photoreactor. No references are given to the use of multifunctional photo/thermocatalytic media of the Type III-V configuration or the use of decoupled media and processes similar to those disclosed here.
U.S. Pat. No. 5,638,589 to de Lasa et al. as previously referenced describes a photocatalytic reactor that requires fiberglass mesh supported photocatalyst wherein only polluted water passes through and treated. The fiberglass mesh is substantially inorganic compound and not a carbon containing synthetic polymeric or biopolymeric material that enhances destruction of pollutants, de Laca et al. describe no separate series connection of different reactors, nor parallel connections of the reactors, nor different length of catalytic media. Furthermore, the conical baskets do not allow for maximum or uniform collection and distribution of the light source photons. Finally, de Lasa et al. has no teaching for thermocatalytic or combined thermo- and photocatalytic media and reactor applications. There are no references to decoupling phenomena and means to mitigate that effect in U.S. Pat. No. 5,638,589.
U.S. Pat. No. 5,580,461 to Cairns et al. teaches a process for treating a fluid comprising at least one chemical contaminant. Their purification process involves first contacting the contaminated fluid with a particulate adsorbent material to adsorb the target compound. The contaminant-loaded adsorbent is then separated from the fluid and brought into contact with aqueous slurry of a suitable photocatalyst. The contaminant on the adsorbent material is decomposed to form a product. The product of photocatalytic decomposition is then removed from the adsorbent material and slurry solution. The regenerated adsorbent material and photocatalyst slurry is recycled. The macro-process described by Cairns et al. employs a combined Type 0 process, does not teach a photoreactor design and the approach is substantially different from the reactors/processes disclosed here. There are no references made to decoupling.
U.S. Pat. No. 5,564,065 to Fleck et al. teaches a reaction chamber which is filled with a fine fibrous material capable of holding powdered titania. At the center of the chamber is a source of ultraviolet light. Air containing carbon monoxide is passed through the reaction chamber to be oxidized into carbon dioxide, which then removed out of the filter. An alternative embodiment uses a rectangular plate several feet square containing fibrous material and TiO2. The reactor design for this application is similar to that of U.S. Pat. No. 5,126,111 to Al-Ekabi et al. The process is substantially a Type I media application with the Category I radiation field. No description is given regarding the use of multifunctional photo- and thermocatalytic media having Class III-V configuration. No references are given to the coupling phenomena or methods to deal with that effect.
U.S. Pat. No. 5.374,405 to Firnberg et al. teaches a rotating fluidized bed reactor in which inert solid particles are held in place by centrifugal force. The reactor includes a rotating porous bed drum within a plenum vessel. Gas enters through the walls of the drum and exits at the top. An ultraviolet light source is included within the drum for effecting photochemical reactions. In one embodiment, the solid particles are inert and loaded with reactant, which react with the gas. In other embodiments of this disclosure, the particles do not contain the reactant and reactant is provided within the gas stream. No references are given to the use of medium-pressure mercury lamp in conjunction with the multifunctional photo/thermocatalytic media of the Type III and V. No description of the decoupling of process energy efficiency from contaminants DRE is given. No direct reference to the use of bandgap semiconductor photocatalysts such as titania or use of high-power lamps are disclosed.
U.S. Pat. No. 5,246,737 to Muradov teaches a method for immobilizing a semiconductor or noble metal material on a number of supports including biopolymers. A solution containing methylene chloride and silicone polymer mixed with titania catalyst was used to form slurry. The slurry was applied onto the surface of cotton fiber with a soft brush. No description is given for treating airborne contaminants. Moreover, Muradov, does not teach a process or photoreactor to accomplish vapor-phase detoxification. Also, the application of photocatalyst in solution with a solvent containing silicone can adversely affect photocatalyst activity toward oxidative mineralization of environmental pollutants. No references are made to the use of multifunctional photo- and thermocatalytic media of the Type III-V configuration. Also, there is no mention of the use of decoupled media or processes similar to those disclosed here.
U.S. Pat. Nos. 4,966,759, 4,892,712 and 5,032,241 to Robertson et al. and U.S. Pat. No. 5,126,111 to Al-Ekabi et al. describe methods for immobilizing TiO2 and other photoactive compounds onto a porous, filamentous, fibrous/stranded glassy transparent mesh for ex-situ oxidation and removal of organic pollutants from fluids. Like U.S. Pat. No. 5,035,784 to Anderson, these are also based on Type II photocatalyst/support and photo-processes. The mesh/matrix can be fiberglass material that supports the sol-gel deposited titania photocatalyst, Robertson et al. correctly recognized usefulness of dispersing the photocatalyst uniformly throughout the reaction volume in much the same way titania slurry is prepared. They also recognized that in a practical slurry-free process. TiO2 must be immobilized onto a suitable transparent support to allow UV transmission and uniform catalyst illumination. The manner in which fiberglass-supported titania is meshed and wrapped around the UV lamp does not produce a well-defined catalytic media that is reproducible and permit uniform catalyst surface irradiance. It is abundantly clear from the previous discussions that a glassy mesh type photocatalytic matrix/media does not readily allow for uniform surface irradiance like the Category I media and photoreactor design. Also, Robertson et al. and Al-Ekabi et al. provide no references to the use of multifunctional photo- and thermocatalytic media with Class III-V configuration and no references are made to decoupled reactor/process designs disclosed here.
U.S. Pat. No. 5,069,885 to Ritchie teaches an apparatus for purification of water in a tubular photoreactor that includes a non-transparent substrate coiled longitudinally and helically around a transparent sleeve. The non-transparent substrate has photocatalyst media bonded to it. Like U.S. Pat. No. 5,035,784 to Anderson, this is also Type II media, Category I radiation field. No references are given to multifunctional photo- and thermocatalytic media of Class III-V configurations. No description of the coupling phenomena and methods to mitigate that are given or discussed.
U.S. Pat. No. 5,045,288 to Raupp et al. describes a technique for removing halogenated volatile and non-volatile organic contaminants from a gaseous stream by mixing with a gaseous oxygen bearing substance in the presence of a solid metal oxide catalyst, exposed to near ultraviolet (UV) radiation. This patent has a Type I photocatalyst/support configuration. Raupp et al. does not teach a photoreactor design or mention polyfunctional catalysts like those disclosed here. No references to the coupling phenomena and methods to mitigate that are given.
U.S. Pat. No. 5,035,784 to Anderson et al. teaches a method for the degradation of complex organic molecules, such as polychlorinated biphenyls on porous titanium ceramic membranes by photocatalysis under ultraviolet light. A special membrane preparation technique known as xe2x80x9csol-gelxe2x80x9d process is used. An organometallic titanium compound is hydrolyzed to form a soluble intermediate, which then condenses into the organic titanium polymer. The process includes the preparation of a particulate gel, which is fired to achieve a ceramic material. Anderson et al. note that the control of process parameters is crucial, one important factor being the sintering temperatures at or below 500xc2x0 C. to give a hard dry ceramic. It is not possible, nor desirable to deposit/immobilize ceramic like membranes atop surfaces of polymeric, biopolymeric (e.g. wood, paper, etc.) origin subject to the very high sol-gel preparation temperatures that will undoubtedly destroy the substrate. The photocatalyst/support arrangement is substantially Type II configuration. The patent by Anderson et al. does not teach a photoreactor design or mention the use of multifunctional catalysts similar to those disclosed here. No references are made to the coupling phenomena and techniques to mitigate that.
U.S. Pat. No. 4,966,665 to Ibusuki et al. describes an application involving vapor-phase, TiO2 based photocatalysis of process vent gases containing chlorinated VOCs such as trichloroethylene (TCE) and tetrachloroethylene, is substantially a Type I photocatalyst/support application. No references are made to the use of multifunctional media having Type III-V configuration or the decoupled reactor designs similar to those disclosed here.
U.S. Pat. No. 4,446,236 to Clyde teaches a photochemical reactor which is divided into a first section suitable for containing a volume of fluid and a second section having at least one light transmitting wall. A porous, high surface area, fiber webbing is mounted within the reactor so that a portion of the webbing is immersed in the fluid to be reacted. The webbing moves within the reactor so that the webbing is sequentially immersed in the fluid contained in the first reactor section and then moved to the second reactor section where the webbing and fluid therein are irradiated. This process is substantially a Type 0 application and Category I radiation field design. Furthermore, no reference is given to mitigating the coupling effect present.
U.S. Pat. No. 3,781,194 to Juillet et al. teaches an application involving vapor-phase photocatalysis using TiO2 in a manner similar to the U.S. Pat. No. 5,045,288 by Raupp et al. The only difference between this patent and the one described above is that Juillet et al. teach a method for oxidizing hydrocarbons to produce aldehydes and ketones, while, Raupp and Dibble describe a similar method for oxidizing halogenated organic compounds such as TCE.
A primary object of the invention is to provide a photoprocess and apparatus for an energy efficient mineralization and detoxification of organic pollutants or undesirable chemicals in both gaseous and aqueous streams.
A secondary object of this invention is to provide apparatus and teach methods of treating contaminated fluids using catalysts and energy sources capable of exciting and activating those catalysts. The energy sources capable of exciting and activating the catalysts include, among others. mercur, vapor lamps (low, medium and high pressure, blacklight and fluorescent light and actinic), xenon lamps (including xenon-mercury and xenon flashlamp) and halogen lamps. In general, these light sources fall into two distinct classes, namely, low- and high-power lamps. The catalyst can be a unifunctional, multifunctional or combination of several unifunctional catalysts. Chemical composition, materials of choice and physical configuration of the catalyst is so chosen to be compatible with the choice of the light source and allow its efficient implementation in the decoupled reactors (full and partial) and traent processes of the present invention. Both low-flux and high-flux media and reactors are based on well-developed principles that include:
(i) Fluid passage with no mass transfer intrusions.
(ii) Uniform irradiance over all catalytically active surface layers.
(iii) Decoupled process energy efficiency from the DRE of target contaminants.
(iv) Utilization of both photons and process waste heat by using multifunctional media.
(v) Simple and readily scaleable photoreactor/photoprocess design.
A third object of the invention is to provide an energy efficient photoprocess and apparatus wherein the catalyst is bonded to the fabric of the base material (i.e. flexible stocking or rigid, metallic or ceramic screen).
A fourth object of this invention is to construct a flexible base material, hereafter called xe2x80x9cstockingxe2x80x9d substantially from a natural polymeric (biopolymeric), synthetic polymeric or a combination of both natural and synthetic polymeric material to which a suitable photocatalyst is firmly applied. It is another object of this invention to expose the catalytic stocking to radiation in the range of wavelengths from 184 to 400 nanometers.
A fifth object of the invention is to fabricate the rigid metallic base material, hereafter called xe2x80x9csupportxe2x80x9d substantially from any suitable metal, metal oxide or an alloy such as 316 or 304 stainless steel.
A sixth object of this invention is to surround the light source with either stocking or the support on to which a suitable photocatalytic, thermocatalytic or a combination of photo- and thermocatalytic material has been deposited, called hereafter xe2x80x9clow-flux catalytic media.xe2x80x9d
A seventh object of the invention is to allow the contaminant stream to pass through the low-flux media, substantially in lateral direction, in a manner that permits retention of the target species within the low-flux catalytic media in a most efficient manner.
An eighth object of the invention is to promote full mineralization of the primary (target species) and secondary reactants to innocuous final products. The plurality of a light source radiating at the above-mentioned wavelength range and the low-flux catalytic media surrounding the light source, axisymmetrically, is referred to hereafter xe2x80x9csingle photocell arrangementxe2x80x9d.
A ninth object of the invention is to provide a flow regime through the single photocell arrangement that minimizes mass transfer intrusions to the low-flux media.
A tenth object of this invention is to provide an optimum configuration that allows most efficient radiant exchange from the light source to the low-flux media and most uniform catalyst surface irradiance.
An eleventh object of the present invention is to provide a segmented low-flux catalytic media; hereafter referred to as xe2x80x9clow-flux multi-stage mediaxe2x80x9d that allows multiple passage of the contaminated stream through the low-flux photocatalytic, thermocatalytic or combined photo- and thermocatalytic media.
A twelfth object of the invention is to segment the low-flux photocatalytic media in a single photocell arrangement in a manner that either maximizes the quantum efficiency of the photoprocess or minimizes the pressure drop across the single photocell, i.e., the difference between the pressures measured at exit port and inlet port of the single photocell unit.
A thirteenth object of the present invention is to provide a novel gas-solid contacting scheme and photoreactor (photocell) design that is most suited for use with the single-stage and multi-stage, low-flux media based on the band-gap photocatalysts, i.e. single-stage and multi-stage photocatalytic media.
A fourteenth object of this invention is to arrange several of these photocatalytic media, in parallel together, each with its own dedicated ultraviolet light source within an integrated reaction vessel, hereafter called xe2x80x9cphotocatalytic bankxe2x80x9d.
A fifteenth object of the invention is to connect/plumb together a number of banks in series to form a xe2x80x9cphotocatalytic modulexe2x80x9d.
A sixteenth object of this invention is to connect/plumb together a number of photocatalytic modules, in parallel or in series, to form a photocatalytic pollution control xe2x80x9cunitxe2x80x9d or PPCU.
A seventeenth object of the invention is to arrange and plumb the sub-units of the PPCU in such a manner that either maximizes the overall energy efficiency (apparent quantum efficiency or photoefficiency) of the photocatalytic unit or minimizes the pressure drop across the photocatalytic unit (i.e. the difference between the exit port and inlet port pressure).
The subject inventor has determined in the subject invention if a linear light source (e.g. a low- or medium-pressure mercury vapor lamp) is used, then the best catalytic media arrangement will be one having a cylindrical (tubular) configuration. Within that configuration, the UV lamp is placed most advantageously along the media axis. It is also desirable to minimize the number of light blocking internals such as baffles, fins, turbulators, pleats, ribs, etc. As such, the active surface of the catalytic media would receive the most uniform irradiance. In the case of high power lamps such as medium- and high-pressure mercury vapor lamps, the type and configuration of the photocatalyst/support (media) is even more critical. This is so because the high power lamps emit radiation and heat at a level orders of magnitude higher than the low-pressure mercury lamps (LPMLs). The output power of a typical commercial LPML is approximately 1 W/in. On the other hand, medium-pressure mercury lamps (MPMLS) are commercially available with power output of up to 300 W/in, nominal. For the irradiance at the photocatalyst surface to remain at or near qEK, a minimum distance, lEK, between the light source and the catalyst surface must be maintained. lEK is a design parameter and characteristic of the type of UV light source used in the photoreactor. In the case of a tubular catalytic media irradiated with a single low-, or medium-pressure mercury lamp, lEK is calculated to be approximately 3.8 inches and 68 feet, respectively. For calculating lEK, the electric to UV light energy conversion efficiency of 0.3 and 0.15 has been assumed for standard LPML and MPML (300 W/in), respectively.
Clearly, based on the lEK calculations determined by the subject inventor, the implementation of LPMLs as the source of UV radiation in practical photoreactors should not be unusually difficult as long as provisions are made to ensure uniform irradiance over all catalytic surfaces. In other words, LPML-driven systems are generally simpler to design and can accommodate many different types of media and reactor configurations. Thus, the primary consideration in constructing an LPML-based photoprocess is to engineer a uniform irradiance over all catalytic surfaces and design for maximum energy efficiency. The essential feature of such an energy efficient photosystem design is decoupling of the process photo-efficiency from conversion efficiency (or DRE) of the target contaminants. Accordingly, it is an object of this invention to provide a novel and improved LPML-based photocatalytic media (hereafter called xe2x80x9clow-flux mediaxe2x80x9d) and a photosystem design that is highly energy efficient. The novel features of such a design will be disclosed later in this document.
Unlike, LPML-driven photo-processes, MPML-based systems, as indicated by the lEK calculation, require large and unrealistic photoreactor dimensions to accommodate both the photocatalyst and the light source. The requirements of very large catalyst surface area, optimum surface irradiance, uniformity of light distribution and media thermal management in MPML-based photo-processes pose a real design challenge. Therefore, it is clear that most photocatalyst/support materials and media configurations of the prior art are not particularly useful for the MPML-based photoreactors. Thus, another object of the present invention is to provide a new and novel method and process for implementing high power light sources for photo- and thermocatalytic service that is compact and highly energy efficient. The approach is based on the use of transition metal aerogel supported catalytic media and others within a specially designed photoreactor. In the terminology of the present application, MPML-based processes and media hereafter termed as the xe2x80x9chigh-fluxxe2x80x9d processes and media.
For high-flux applications, a rotating fluidized bed photoreactor is most desirable. The photocatlytic media is in the form of multifunctional, moderate temperature catalysts of the Type III (e.g. metal oxide aerogels, co-gelled metal oxide aeroaels including titania-silica aerogels and transition metal aerogel-supported catalysts, etc.) or Type V (e.g. supported transition metal oxide catalysts, cation modified zeolites and doped titania catalyst). The reactor consists of a porous rotating drum located within a stationary plenum vessel. The waste stream enters the rotating drum through the porous side wall of the drum and exits from an opening near the top. Rate of the rotation of the drum and amount of solids added and bed thickness is adjusted to minimize bed carry over and maintain operation at or near minimum fluidization condition wherein the bed material expands but few bubbles are formed within the bed A medium pressure mercury lamp placed within a quartz or fused silica sleeve at the middle and inserted into the photoreactor from the bottom or top. Provisions are made to allow feeding and removal of the photocatalytic media during normal reactor operation, if necessary.
Therefore, other objects of the invention described here are to provide gas-phase photocatalysis and air purification system with very high process quantum efficiency for treating various organic contaminants including: aliphatics, aromatics, halogenated organics, mercaptants, sulfur gases, and others.
Further objects and advantages of this invention will be apparent from the following detailed description of the preferred embodiments, which are illustrated schematically in the accompanying drawings.